Cerium oxide abrasive and method of polishing substrates

ABSTRACT

This invention provides a cerium oxide abrasive with which the surfaces of substrates such as SiO 2  insulating films can be polished at a high rate without causing scratches. The abrasive of the present invention comprises a slurry comprising cerium oxide particles whose primary particles have a diameter of from 10 nm to 600 nm and a median diameter of from 30 nm to 250 nm and slurry particles have a median diameter of from 150 nm to 600 nm and a maximum diameter of 3,000 nm or smaller, the cerium oxide particles being dispersed in a medium.

TECHNICAL FIELD

This invention relates to a cerium oxide abrasive and a method ofpolishing substrates.

BACKGROUND ART

In semiconductor device fabrication processes, colloidal silica typeabrasives have commonly been studied as chemomechanical abrasives forsmoothing inorganic insulating film layers such as SiO₂ insulating filmsformed by a process such as plasma-assisted CVD (chemical vapordeposition) or low-pressure CVD. Colloidal silica type abrasives areproduced by growing silica particles into grains by, e.g., thermaldecomposition of silicon tetrachloride, followed by pH-adjustment withan alkali solution containing non alkali metal, such as aqueous ammonia.Such abrasives, however, have a technical problem of a low polishingrate which prevents them being put into practical use, because theinorganic insulating films can not be polished at a sufficiently highpolishing rate.

Meanwhile, cerium oxide abrasives are used in glass-surface polishingfor photomasks. The cerium oxide abrasives are useful for finish mirrorpolishing because they have a hardness lower than silica particles andalumina particles and hence the polishing surface is very difficult toscratch. Also, cerium oxide has a chemically active nature and is knownas a strong oxidizing agent. Making the most of this advantage, it isuseful to apply the cerium oxide in chemomechanical abrasives forinsulating films. However, if the cerium oxide abrasives used inglass-surface polishing for photomasks are used in the polishing ofinorganic insulating films as they are, they have so large a primaryparticle diameter that the insulating film surface may come to havepolish scratches which are visually perceivable.

DISCLOSURE OF THE INVENTION

The present invention provides a cerium oxide abrasive that can polishthe surfaces of objects such as SiO₂ insulating films without causingscratches and at a high rate, and also provides a method of polishingsubstrates.

The cerium oxide abrasive of the present invention comprises a slurrycomprising cerium oxide particles whose primary particles have a mediandiameter of from 30 nm to 250 nm and slurry particles have a mediandiameter of from 150 nm to 600 nm; the cerium oxide particles beingdispersed in a medium.

The cerium oxide abrasive of the present invention may also comprise aslurry made up of cerium oxide particles whose primary particles have amedian diameter of from 100 nm to 250 nm and slurry particles have amedian diameter of from 150 nm to 350 nm; the cerium oxide particlesbeing dispersed in a medium.

In the above cerium oxide particles, the primary particles maypreferably have a maximum diameter of 600 nm or smaller and aprimary-particle diameter of from 10 nm to 600 nm.

The cerium oxide abrasive of the present invention may still furthercomprise a slurry made up of cerium oxide particles whose primaryparticles have a median diameter of from 30 nm to 70 nm and slurryparticles have a median diameter of from 250 nm to 600 nm; the ceriumoxide particles being dispersed in a medium.

The above cerium oxide particles may preferably have a primary-particlediameter of from 10 nm to 100 nm.

In the cerium oxide abrasive of the present invention, the cerium oxideparticles may preferably have a maximum particle diameter of 3,000 nm orsmaller.

Water may be used as the medium, and at least one dispersant selectedfrom a water-soluble organic high polymer, a water-soluble anionicsurface-active agent, a water-soluble nonionic surface-active agent anda water-soluble amine may be used, of which ammonium polyacrylate ispreferred.

As the cerium oxide particles, cerium oxide obtained by firing ceriumcarbonate may preferably be used.

The cerium oxide abrasive of the present invention can polish a givensubstrate for semiconductor chips or the like, on which silica filmshave been formed.

BEST MODES FOR PRACTICING THE INVENTION

The cerium oxide is commonly obtained by firing a cerium compound suchas cerium carbonate, cerium sulfate or cerium oxalate. SiO₂ insulatingfilms formed by TEOS-CVD, for example, can be polished at a higher rateas abrasives have a larger primary-particle diameter and a lower crystalstrain, i.e., have better crystallinity, but tend to be prone to polishscratches. Accordingly, the cerium oxide particles used in the presentinvention are produced without making their crystallinity so high. Also,since the abrasive may be used to polish semiconductor chips, itscontent of alkali metals and halogens may preferably be controlled to be1 ppm or less.

The abrasive of the present invention has a high purity, and does notcontain more than 1 ppm of Na, K, Si, Mg, Ca, Zr, Ti, Ni, Cr and Fe eachand more than 10 ppm of Al.

In the present invention, firing may be employed as a process forproducing the cerium oxide particles. In particular, low-temperaturefiring is preferred, which can make the crystallinity as low as possiblein order to produce particles that do not cause polish scratches. Sincethe cerium compounds have an oxidation temperature of 300° C., they maypreferably be fired at a temperature of from 600° C. to 900° C.

The cerium carbonate may preferably be fired at a temperature of from600° C. to 900° C. for 5 to 300 minutes in an oxidative atmosphere ofoxygen gas or the like.

The cerium oxide obtained by firing may be pulverized by dry-processpulverization such as jet milling or by wet-process pulverization suchas bead milling. The jet milling is described in, e.g., KAGAKU KOGYORONBUNSHU (Chemical Industry Papers), Vol. 6, No. 5 (1980), pages527-532. Cerium oxide obtained by firing was pulverized by dry-processpulverization such as jet milling, whereupon a pulverization residue wasseen to occur.

The slurry of cerium oxide in the present invention is obtained bydispersion-treating an aqueous solution containing cerium oxideparticles produced in the manner described above or a compositioncomprising cerium oxide particles collected from this aqueous solution,water and optionally a dispersant. Here, the cerium oxide particles maypreferably be used in a concentration ranging, but not particularlylimited to, from 0.1 to 10% by weight in view of readiness to handlesuspensions. As the dispersing agent, it may include, as thosecontaining no metal ions, water-soluble organic high polymers such asacrylic polymers and ammonium salts thereof, methacrylic polymers andammonium salts thereof, and polyvinyl alcohol, water-soluble anionicsurface-active agents such as ammonium lauryl sulfate and ammoniumpolyoxyethylene lauryl ether sulfate, water-soluble nonionicsurface-active agents such as polyoxyethylene lauryl ether andpolyethylene glycol monostearate, and water-soluble amines such asmonoethanolamine and diethanolamine.

Ammonium polyacrylate, in particular, ammonium polyacrylate havingweight-average molecular weight of from 5,000 to 20,000 is preferred.Any of these dispersing agents may preferably be added in an amountranging from 0.01 part by weight to 5 parts by weight based on 100 partsby weight of the cerium oxide particles in view of the dispersibilityand anti-sedimentation properties of particles in the slurry. In orderto improve its dispersion effect, the dispersing agent may preferably beput in a dispersion machine simultaneously with the particles at thetime of dispersion treatment.

These cerium oxide particles may be dispersed in water by dispersiontreatment using a conventional agitator, and besides by using ahomogenizer, an ultrasonic dispersion machine or a ball mill.Particularly for dispersing the cerium oxide particles as fine particlesof 1 μm or smaller, it is preferable to use wet-process dispersionmachines such as a ball mill, a vibration ball mill, a satellite ballmill and a media agitating mill. In a case where the slurry should bemade more highly alkaline, an alkaline substance containing no metalions, such as aqueous ammonia, may be added during the dispersiontreatment or after the treatment.

The cerium oxide abrasive of the present invention may be used as it isin the form of the above slurry. It may also be used as an abrasive towhich an additive such as N,N-diethylethanolamine,N,N-dimethylethanolamine or aminoethylethanolamine has been added.

Primary particles constituting the cerium oxide particles dispersed inthe slurry of the present invention have a median diameter of from 30 to250 nm and their particles standing dispersed (slurry particles) have amedian diameter of from 150 to 600 nm.

If the primary particles have a median diameter smaller than 30 nm orthe particles standing dispersed have a median diameter smaller than 150nm, the surfaces of objects to be polished such as SiO₂ insulating filmscan not be polished at a high rate. If the primary particles have amedian diameter larger than 250 nm or the slurry particles have a mediandiameter larger than 600 nm, scratches may occur on the surfaces ofobjects to be polished such as SiO₂ insulating films.

Cerium oxide particles whose primary particles have a median diameter offrom 100 nm to 250 nm and slurry particles having a median diameter offrom 150 nm to 350 nm are preferred. If their respective mediandiameters are smaller than the above lower-limit values, a low polishingrate may result and, if they are larger than the above upper-limitvalues, scratches tend to occur.

In the above cerium oxide particles, the primary particles maypreferably have a maximum diameter not larger than 600 nm, and maypreferably have a primary-particle diameter of from 10 to 600 nm. Theprimary particles having a particle diameter larger than the upper-limitvalue 600 nm may result in occurrence of scratches and those having aparticle diameter smaller than 10 nm may result in a low polishing rate.

Cerium oxide particles whose primary particles have a median diameter offrom 30 nm to 70 nm and slurry particles having a median diameter offrom 250 nm to 600 nm are also preferred. If their respective mediandiameters are smaller than the above lower-limit values, a low polishingrate may result and, if they are larger than the above upper-limitvalues, scratches tend to occur.

In the above cerium oxide particles, the primary particles maypreferably have a diameter of from 10 to 100 nm. If the primaryparticles have a particle diameter smaller than 10 nm, a low polishingrate may result. If they have a particle diameter larger than theupper-limit value 100 nm, scratches tend to occur.

In the cerium oxide abrasive of the present invention, the cerium oxideparticles may preferably have a maximum diameter not larger than 3,000nm. If the cerium oxide particles have a maximum diameter larger than3,000 nm, scratches tend to occur.

The cerium oxide particles obtained by pulverizing fired cerium oxide bydry-process pulverization such as jet milling contains a pulverizationresidue. Such pulverization residue particles differ from agglomeratesof primary particles having re-agglomerated, and are presumed to bebroken by stress at the time of polishing to generate active surfaces,which are considered to contribute to the polishing of the surfaces ofobjects to be polished, such as SiO₂ insulating films, at a high ratewithout causing scratches.

The slurry of the present invention may contain pulverization residueparticles having a particle diameter of 3,000 nm or smaller.

In the present invention, the primary-particle diameter is measured byobserving the particles on a scanning electron microscope (e.g., ModelS-900, manufactured by Hitachi, Ltd.). The particle diameter of thecerium oxide particles as slurry particles is measured by laserdiffraction (using, e.g., MASTER SIZER MICROPLUS, manufactured byMalvern Instruments Ltd.; refractive index: 1.9285; light source: He—Nelaser; absorption: 0).

The primary particles constituting the cerium oxide particles dispersedin the slurry of the present invention may preferably have an aspectratio of from 1 to 2 and a median value of 1.3. The aspect ratio ismeasured by observing the particles on a scanning electron microscope(e.g., Model S-900, manufactured by Hitachi Ltd.).

As the cerium oxide particles to be dispersed in the slurry of thepresent invention, cerium oxide particles whose structural parameter Ywhich represents an isotropic microstrain in analysis by the powderX-ray Rietvelt method (RIETAN-94) has a value of from 0.01 to 0.70 maybe used. The use of cerium oxide particles having such a crystal strainmakes it possible to carry out polishing without scratching the surfacesof objects and also at a high rate.

The cerium oxide particles dispersed in the slurry of the presentinvention may preferably have a specific surface area of from 7 to 45m²/g. Those having a specific surface area smaller than 7 m²/g tend tomake scratches on the surfaces of polish objects, and those having aspecific surface area larger than 45 m²/g tend to result in a lowpolishing rate. The specific surface area of the cerium oxide particlesof the slurry is identical with the specific surface area of ceriumoxide particles to be dispersed.

The cerium oxide particles in the slurry of the present invention maypreferably have a zeta potential of from −100 mV to −10 mV. This bringsabout an improvement in dispersibility of the cerium oxide particles andmakes it possible to carry out polishing without scratching the surfacesof polish objects and also at a high rate.

The cerium oxide particles dispersed in the slurry of the presentinvention may have an average particle diameter of from 200 nm to 400 nmand a particle size distribution half width of 300 nm or smaller.

The slurry of the present invention may preferably have a pH of from 7to 10, and more preferably from 8 to 9.

After the slurry has been prepared, it may be put in a container ofpolyethylene or the like and left at 5 to 55° C. for 7 days or more, andmore preferably 30 days or more, so that the slurry may cause lessscratches.

The slurry of the present invention has such good dispersion and such alow rate of sedimentation that its rate of change in concentration after2-hour leaving is less than 10% at every height and every position of acolumn of 10 cm in diameter and 1 m in height.

The inorganic insulating films on which the cerium oxide abrasive of thepresent invention is used may be formed by a process includinglow-pressure CVD and plasma-assisted CVD. The formation of SiO₂insulating films by low-pressure CVD makes use of monosilane SiH₄ as anSi source and oxygen O₂ as an oxygen source. Oxidation reaction of thisSiH₄—O₂ system to obtain the inorganic insulating film may be carriedout at a low temperature of about 400° C. or below. When phosphorus (P)is doped in order to make the surface smooth by high-temperaturereflowing, it is preferable to use a reaction gas of SiH₄—O₂—PH₃ system.The plasma-assisted CVD has an advantage that any chemical reactionwhich requires a high temperature under normal heat equilibrium can becarried out at a low temperature. Plasma may be generated by a processincluding two types of coupling, namely capacitive coupling andinductive coupling. Reaction gas may include gases of SiH₄—N₂O systemmaking use of SiH₄ as an Si source and N₂O as an oxygen source and gasesof TEOS-O₂ system making use of tetraethoxysilane (TEOS) as an Si source(i.e., TEOS plasma-assisted CVD method). Substrate temperature maypreferably be within the range of from 250° C. to 400° C., and reactionpressure from 67 Pa to 400 Pa. Thus, the SiO₂ insulating films in thepresent invention may be doped with an element such as phosphorus orboron.

As the given substrate, substrates may be used which are obtained byforming SiO₂ insulating films on semiconductor substrates, i.e.,semiconductor substrates such as a semiconductor substrate at the stagewhere circuit elements and wiring patterns have been formed thereon or asubstrate at the stage where circuit elements have been formed thereon.The SiO₂ insulating film formed on such a semiconductor substrate ispolished with the cerium oxide abrasive described above, whereby anyunevenness on the SiO₂ insulating film surface can be removed to providea smooth surface over the whole area of the semiconductor substrate.Here, as a polishing apparatus, commonly available polishing apparatusmay be used, having i) a holder for holding a semiconductor substrateand ii) a platen (provided with a motor whose number of revolution isvariable) on which a polishing cloth (a pad) is stuck. As the polishingcloth, commonly available nonwoven fabric, foamed polyurethane or porousfluorine resin may be used, and there are no particular limitations. Thepolishing cloth may also preferably be processed to provide grooveswhere the slurry may gather. There are no particular limitations onpolishing conditions, and preferably the platen may be rotated at asmall number of revolution of 100 rpm or below so that the semiconductorsubstrate may not run out. Pressure applied to the semiconductorsubstrate may preferably be 1 kg/cm or below so that the substrate doesnot get scratched as a result of polishing. In the course of polishing,the slurry is fed continuously to the polishing cloth by means of a pumpor the like. There are no particular limitations on the feed rate ofthis slurry. It is preferable for the surface of the polishing cloth toalways be covered with the slurry.

Semiconductor substrates on which the polishing has been completed maypreferably be well rinsed in running water and thereafter water dropsadhering to the surfaces of semiconductor substrates are brushed off bymeans of a spin dryer or the like, followed by drying. On the SiO₂insulating film having been thus smoothed, second-layer aluminum wiringis formed. An SiO₂ insulating film is again formed between the wiringand on the wiring, followed by polishing with the cerium oxide abrasivedescribed above, whereby any unevenness on the insulating film surfaceis removed to provide a smooth surface over the whole area of the semiconductor substrate. This process may be repeated a given number oftimes so that a semiconductor having the desired number of layers can beproduced.

The cerium oxide abrasive of the present invention may be used to polishnot only the SiO₂ insulating films formed on semiconductor substrates,but also SiO₂ insulating films formed on wiring boards having givenwiring, glass, inorganic insulating films such as silicon nitride film,optical glass such as photomasks, lenses and prisms, inorganicconductive films such as ITO (indium tin oxide) film, optical integratedcircuits, optical switching devices or optical waveguides which areconstituted of glass and a crystalline material, optical fiber endfaces, optical single crystals such as scintillators, solid-state lasersingle crystals, blue-laser LED (light-emitting diode) sapphiresubstrates, semiconductor single crystals such as SiC, GaP and GaAs,magnetic disk glass substrates, magnetic heads and so forth.

Thus, in the present invention, what is referred to as the givensubstrate includes semiconductor substrates on which SiO₂ insulatingfilms have been formed, wiring boards on which SiO₂ insulating filmshave been formed, glass, inorganic insulating films such as siliconnitride film, optical glasses such as photomasks, lenses and prisms,inorganic conductive films such as ITO film, optical integratedcircuits, optical switching devices or optical waveguides which areconstituted of glass and a crystalline material, optical fiber endfaces, optical single crystals such as scintillators, solid-state lasersingle crystals, blue-laser LED sapphire substrates, semiconductorsingle crystals such as SiC, GaP and GaAs, magnetic disk glasssubstrates, and magnetic heads.

The slurry prepared by dispersing the cerium oxide particles in themedium reacts chemically with part of an insulating film layer providedon the given substrate to form a reactive layer, and the reactive layeris removed mechanically with the cerium oxide particles, thus making itpossible to polish the insulating film layer at a high rate and alsowithout causing any polish scratches.

Cerium oxide abrasives are used in glass-surface polishing forphotomasks. The cerium oxide abrasives are useful for finish mirrorpolishing because they have a lower hardness than silica particles andalumina particles and hence the polishing surface is unlikely to bescratched. Also, cerium oxide has a chemically active nature and isknown as a strong oxidizing agent. Making the most of this advantage, itis useful for the cerium oxide to be applied in chemomechanicalabrasives for insulating films. However, if the cerium oxide abrasivesused in glass-surface polishing for photomasks are used in the polishingof insulating films as they are, the particles have such highcrystallinity that the insulating film surface may be subject to polishscratches which are visually perceivable.

Factors that determine the crystallinity include crystallite size andcrystal strain. When the crystallite size is as large as 1 μm or more,polish scratches tend to occur. Also, even when the crystallite size issmall, polish scratches may occur if particles having no crystal strainare used. However, some cerium oxide particles have too low acrystallinity to cause any polish scratches, but are not able to effecthigh-rate polishing. Thus, cerium oxide particles which make it possibleto prevent polish scratches and to effect high-rate polishing have arange of proper particle size and a proper degree of strain. Factorsthat determine the polishing rate include not only the crystallinity ofparticles stated above but also the active chemical nature inherent incerium oxide.

Even with use of silica particles having a higher particle hardness thanthe cerium oxide particles, silica slurries have a much lower polishingrate than the cerium oxide slurry. This indicates that the cerium oxideslurry has a stronger chemical factor in the chemomechanical polishing.The surface of SiO₂ insulating film stands charged negatively in asolution having a hydrogen ion concentration of pH 3 or more. Whenpolished with a slurry making use of cerium oxide particles standingcharged positively, an inert film composed chiefly of cerium oxide isformed. This inert film can not be removed by washing with water, and isremoved using a strongly acidic solution such as nitric acid.Simultaneously with the removal of the inert film by the use of an acid,the insulating layer is removed to an extent of 1,000 nm or more. Theinsulating film thus removed is a reactive layer formed when the inertfilm is formed. The inert film is also formed when the cerium oxideparticles stand charged negatively. The degree of adhesion of the inertfilm to the insulating film depends on how far the particles arecharged. For example, an inert film formed when the absolute negativevalue where the particles stand charged is great can be removed bywashing with water or brush cleaning. That is, the degree of formationof the inert layer and reactive layer depends on the state of chargingof particles. This phenomenon of formation of the inert film is not seenin silica slurries, and is a phenomenon inherent in the cerium oxideslurry. This phenomenon is one of the factors that determine thehigh-rate polishing. The cerium oxide particles scrape off these inertfilm and reactive layer. This phenomenon is the mechanical factor in thechemomechanical polishing. If particles have a low crystallinity, thereactive layer can not be removed, resulting in a low polishing rate. Onthe other hand, particles having a high crystallinity can remove thereactive layer with ease, and can quickly scrape off the reactive layerformed immediately after removal. Thus, the formation of reactive layersand the polishing with particles take place one after another, so thatthe polishing can be carried out at a high rate.

As a method for examining the state of charging of particles in theslurry, the zeta potential measurement is available. To describe itsspecific principle, the cerium oxide slurry is put in a measuring celllike the one provided with platinum electrodes on both sides, and avoltage is applied to the both electrodes. Cerium oxide particles havingcome to have charges upon application of the voltage move toward theelectrode side having a polarity reverse to that of the charges. Themobility thereof is determined, and the zeta potential of particles canbe determined from a known expression of the relationship betweenmobility and zeta potential. To form the inert film and reactive layer,the cerium oxide slurry may preferably have a zeta potential of −100 mVor above. However, when particles are charged positively or, even thoughcharged negatively, have an absolute value smaller than 10 mV, the inertfilm is formed so strongly that the polishing with optimum particlesthat do not cause polish scratches is impossible. Accordingly, theslurry may preferably have a zeta potential of from −100 mV to −10 mV.

Using the cerium oxide abrasive comprising the slurry prepared bydispersing the cerium oxide particles in the medium, an inert film thatmay prevent the polishing proceeding thereon may be formed only on thesurface of one certain type of film and the other film may be polishedselectively, whereby layers formed of two or more types of differentfilms on the substrate can be polished.

The inert film that may prevent the polishing proceeding thereon may beformed only on the surface of one certain type of film among the layersformed of two or more types of different films on the substrate, and thefilm area where such an inert film has been formed, may serve as astopper so that the other film may be polished selectively, whereby thelayers formed as described above can be made smooth.

This polishing method utilizes the properties that the polishing barelyproceeds on the surfaces of a certain interlayer insulating film and acertain interlayer smoothing film because an inert film comprised ofabrasive particles or a reaction product of a polishing liquidcomposition with a film composition is formed on such surfaces. Theinert film refers to a surface layer that may make the polishing ratelower than the film to be polished originally. When such a certaininterlayer insulating film and interlayer smoothing film on which theinert film may be formed are used to form patterns of semiconductorchips, another interlayer film on which the polishing proceeds may beformed as its upper layer, whereby a global smoothness can be achievedusing the lower layer film as a stopper.

Those comprised of such two or more types of different films formed on asubstrate may include those in which the substrate is a semiconductorsubstrate and the layers formed thereon are an organic SOG (spin-onglass) film and an SiO₂ film formed by chemical vapor deposition orthermal oxidation, where the film on which the inert film is formed isthe SiO₂ film and the film to be polished selectively is the organic SOGfilm.

The organic SOG film is a film formed by coating a coating solutionobtained by, e.g., hydrolyzing an alkoxysilane and an alkylalkoxysilanein an organic solvent such as alcohol with addition of water and acatalyst, on a substrate by spin coating or the like, followed byheating to cause the coating to cure.

In such an insulating film, preferred is an insulating film in which thenumber of Si atoms originating from siloxane bonds and the number of Catoms originating from alkyl groups in the insulating film have therelationship of:

Number of C atoms/(number of Si atoms+number of C atoms)≧0.1.

On the organic SOG insulating film layer having been smoothed, aCVD-SiO₂ film and second-layer aluminum wiring are formed, and lowerlayer CVD-SiO₂ film and organic SOG insulating film are formed betweenthe wiring and on the wiring, followed by polishing using the abovecerium oxide slurry to thereby remove unevenness of the insulating filmlayer surface to provide a smooth face over the whole area of thesemiconductor substrate. This process may be repeated a given number oftimes so that a semiconductor having the desired number of layers can beproduced. In the process where the films formed of two or more types offilms are polished to form the intended structure by utilizing thispolishing method, the smoothing that utilizes the selective polishingcan make the process simple and highly precise.

EXAMPLE 1

(Production 1 of cerium oxide particles)

2 kg of cerium carbonate hydrate was placed in a container made ofplatinum, followed by firing at 800° C. for 2 hours in air to obtainabout 1 kg of a yellowish white powder. Phase identification of thispowder was made by X-ray diffraction to confirm that it was ceriumoxide. The fired powder had particle diameters of 30 to 100 μm. Theparticle surfaces of the fired powder were observed on a scanningelectron microscope, where grain boundaries of cerium oxide were seen.Diameters of cerium oxide primary particles surrounded by the grainboundaries were measured to find that the median diameter and maximumdiameter in their particle size distribution were 190 nm and 500 nm,respectively. Precision measurement by X-ray diffraction was made on thefired powder, and the results obtained were analyzed by the Rietveltmethod (RIETAN-94) to find that the value of structural parameter Xwhich represents primary-particle diameter was 0.080 and the value ofstructural parameter Y which represents an isotropic microstrain was0.223. Using a jet mill, 1 kg of the cerium oxide powder was dry-processpulverized. The pulverized particles obtained were observed on ascanning electron microscope to find that large pulverization residueparticles of from 1 μm to 3 μm diameter and pulverization residueparticles of from 0.5 μm to 1 μm diameter were present in a mixed statein addition to small particles having the same size as primary-particlediameter. The pulverization residue particles were not agglomerates ofprimary particles. Precision measurement by X-ray diffraction was madeon the pulverized particles, and the results obtained were analyzed bythe Rietvelt method (RIETAN-94) to find that the value of structuralparameter X which represents primary-particle diameter was 0.085 and thevalue of structural parameter Y which represents an isotropicmicrostrain was 0.264. As the result, there was almost no variation inprimary-particle diameter caused by pulverization and also strains wereintroduced into particles as a result of pulverization. Measurement ofspecific surface area by the BET method also revealed that it was 10M²/g.

(Production 2 of cerium oxide particles)

2 kg of cerium carbonate hydrate was placed in a container made ofplatinum, followed by firing at 750° C. for 2 hours in air to obtainabout 1 kg of a yellowish white powder. Phase identification of thispowder was made by X-ray diffraction to confirm that it was ceriumoxide. The fired powder had particle diameters of 30 to 100 μm. Theparticle surfaces of the fired powder were observed on a scanningelectron microscope, where grain boundaries of cerium oxide were seen.Diameters of cerium oxide primary particles surrounded by the grainboundaries were measured to find that the median diameter and maximumdiameter in their particle size distribution were 141 nm and 400 nm,respectively. Precision measurement by X-ray diffraction was made on thefired powder, and the results obtained were analyzed by the Rietveltmethod (RIETAN-94) to find that the value of structural parameter Xwhich represents primary-particle diameter was 0.101 and the value ofstructural parameter Y which represents an isotropic microstrain was0.223. Using a jet mill, 1 kg of the cerium oxide powder was dry-processpulverized. The pulverized particles obtained were observed on ascanning electron microscope to find that large pulverization residueparticles of from 1 μm to 3 μm diameter and pulverization residueparticles of from 0.5 μm to 1 μm diameter were present in a mixed statein addition to small particles having the same size as primary-particlediameter. The pulverization residue particles were not agglomerates ofprimary particles. Precision measurement by X-ray diffraction was madeon the pulverized particles, and the results obtained were analyzed bythe Rietvelt method (RIETAN-94) to find that the value of structuralparameter X which represents primary-particle diameter was 0.104 and thevalue of structural parameter Y which represents an isotropicmicrostrain was 0.315. As the result, there was almost no variation inprimary-particle diameter caused by pulverization and also strains wereintroduced into particles as a result of pulverization. Measurement ofspecific surface area by the BET method also revealed that it was 16m²/g.

(Production of cerium oxide slurry)

1 kg of the cerium oxide particles obtained in the above production 1 or2, 23 g of an aqueous ammonium polyacrylate solution (40% by weight) and8,977 g of deionized water were mixed, and the mixture formed wassubjected to ultrasonic dispersion for 10 minutes with stirring. Theslurries thus obtained were filtered with a 1 μm filter, followed byfurther addition of deionized water to obtain a 3% by weight abrasive.The slurries had a pH of 8.3.

Particle size distribution of slurry particles was examined by laserdiffraction (measured with a measuring apparatus: MASTER SIZERMICROPLUS, manufactured by Malvern Instruments Ltd.; refractive index:1.9285; light source: He—Ne laser; absorption: 0) to find that themedian diameter was 200 nm for each slurry. With regard to maximumparticle diameter, particles of 780 nm or larger were in a content of 0%by volume.

To examine dispersibility of the slurries and charges of the slurryparticles, the zeta potentials of the slurries were measured. Eachcerium oxide slurry was placed in a measuring cell provided withplatinum electrodes on both sides, and a voltage of 10 V was applied toboth electrodes. Slurry particles having come to have charges uponapplication of the voltage move toward the electrode side having apolarity reverse to that of the charges. The zeta potential of particlescan be determined by determining their mobility. As a result of themeasurement of zeta potential, it was confirmed that the particles ineach slurry were charged negatively, and showed a large absolute valueof −50 mV or −63 mV, respectively, having a good dispersibility.

(Polishing of insulating film layer)

Silicon wafers on which SiO₂ insulating films produced by TEOSplasma-assisted CVD were formed were each set on a holder provided witha suction pad stuck thereon for attaching the substrate to be held, andthe holder was placed, with its insulating film side down, on a platenon which a polishing pad made of porous urethane resin was stuck. Aweight was further placed thereon so as to provide a processing load of300 g/cm².

The platen was rotated at 30 rpm for 2 minutes to polish the insulatingfilm while dropwise adding the above cerium oxide slurry (solid content:3% by weight) onto the platen at a rate of 50 ml/minute. After thepolishing was completed, the wafer was detached from the holder and thenwell rinsed in running water, followed by further cleaning for 20minutes by an ultrasonic cleaner. After the cleaning was completed, thewafer was set on a spin dryer to remove drops of water, followed bydrying for 10 minutes by a 120° C. dryer.

Changes in layer thickness before and after the polishing were measuredwith a light-interference type layer thickness measuring device. As aresult, it was found that as a result of this polishing the insulatingfilms were abraded by 600 nm and 580 nm (polishing rate: 300 nm/minute,290 nm/minute), respectively, and each wafer was in a uniform thicknessover its whole area. The surfaces of the insulating films were alsoobserved using an optical microscope, where no evident scratches wereseen.

EXAMPLE 2

(Production of cerium oxide particles)

2 kg of cerium carbonate hydrate was placed in a container made ofplatinum, followed by firing at 700° C. for 2 hours in air to obtainabout 1 kg of a yellowish white powder. Phase identification of thispowder was made by Xray diffraction to confirm that it was cerium oxide.The fired powder had particle diameters of 30 to 100 μm. The particlesurfaces of the fired powder were observed on a scanning electronmicroscope, where grain boundaries of cerium oxide were seen. Diametersof cerium oxide primary particles surrounded by the grain boundarieswere measured to find that the median diameter and maximum diameter intheir particle size distribution were 50 nm and 100 nm, respectively.Precision measurement by X-ray diffraction was made on the fired powder,and the results obtained were analyzed by the Rietvelt method(RIETAN-94) to find that the value of structural parameter X whichrepresents primary-particle diameter was 0.300 and the value ofstructural parameter Y which represents an isotropic microstrain was0.350.

Using a jet mill, 1 kg of the cerium oxide powder was dry-processpulverized. The pulverized particles obtained were observed on ascanning electron microscope to find that large pulverization residueparticles of from 2 μm to 4 μm diameter and pulverization residueparticles of from 0.5 μm to 1.2 μm diameter were present in a mixedstate in addition to small particles having the same size asprimary-particle diameter. The pulverization residue particles were notagglomerates of primary particles. Precision measurement by X-raydiffraction was made on the pulverized particles, and the resultsobtained were analyzed by the Rietvelt method (RIETAN-94) to find thatthe value of structural parameter X which represents primary-particlediameter was 0.302 and the value of structural parameter Y whichrepresents an isotropic microstrain was 0.412. As the result, there wasalmost no variation in primary-particle diameter to be caused bypulverization and also strains were introduced into particles as aresult of pulverization. Measurement of specific surface area by the BETmethod also revealed that it was 40 m²/g.

(Production of cerium oxide slurry)

1 kg of the cerium oxide particles produced in the above, 23 g of anaqueous ammonium polyacrylate solution (40% by weight) and 8,977 g ofdeionized water were mixed, and the mixture formed was subjected toultrasonic dispersion for 10 minutes with stirring. The slurry thusobtained was filtered with a 2 μm filter, followed by further additionof deionized water to obtain a 3% by weight abrasive. The slurry had apH of 8.0. Particle size distribution of slurry particles was examinedby laser diffraction (measuring apparatus: MASTER SIZER MICROPLUS;refractive index: 1.9285) to find that the median diameter was 510 nm.With regard to maximum particle diameter, particles of 1,430 nm orlarger were in a content of 0%.

To examine dispersibility of the slurry and charges of the slurryparticles, the zeta potential of the slurry was measured. The ceriumoxide slurry was put in a measuring cell provided with platinumelectrodes on both sides, and a voltage of 10 V was applied to bothelectrodes. Slurry particles having come to have charges uponapplication of the voltage move toward the electrode side having apolarity reverse to that of the charges. The zeta potential of particlescan be determined by determining their mobility. As a result of themeasurement of zeta potential, it was confirmed that the particles werecharged negatively, and showed a large absolute value of −64 mV, havinga good dispersibility.

(Polishing of insulating film layer)

A silicon wafer on which an SiO₂ insulating film produced by TEOSplasma-assisted CVD was formed was set on a holder provided with asuction pad stuck thereon for attaching the substrate to be held, andthe holder was placed, with its insulating film side down, on a platenon which a polishing pad made of porous urethane resin was stuck. Aweight was further placed thereon so as to provide a processing load of300 g/cm².

The platen was rotated at 30 rpm for 2 minutes to polish the insulatingfilm while dropwise adding the above cerium oxide slurry (solid content:3% by weight) onto the platen at a rate of 35 ml/minute. After thepolishing was completed, the wafer was detached from the holder and thenwell rinsed in running water, followed by further cleaning for 20minutes using an ultrasonic cleaner. After the cleaning was completed,the wafer was set on a spin dryer to remove drops of water, followed bydrying for 10 minutes using a 120° C. dryer. Changes in layer thicknessbefore and after the polishing were measured with a light-interferencetype layer thickness measuring device. As the result, it was found thatas a result of this polishing the insulating film was abraded by 740 nm(polishing rate: 370 nm/minute) and the wafer was in a uniform thicknessover its whole area. The surface of the insulating film was alsoobserved using an optical microscope, where no evident scratches wereseen.

EXAMPLE 3

(Production of cerium oxide particles)

2 kg of cerium carbonate hydrate was placed in a container made ofplatinum, followed by firing at 800° C. for 2 hours in air to obtainabout 1 kg of a yellowish white powder. Phase identification of thispowder was made by Xray diffraction to confirm that it was cerium oxide.The fired powder had particle diameters of 30 to 100 μm. The particlesurfaces of the fired powder were observed on a scanning electronmicroscope, where grain boundaries of cerium oxide were seen. Diametersof cerium oxide primary particles surrounded by the grain boundarieswere measured to find that the median diameter and maximum diameter intheir particle size distribution were 190 nm and 500 nm, respectively.Precision measurement by X-ray diffraction was made on the fired powder,and the results obtained were analyzed by the Rietvelt method(RIETAN-94) to find that the value of structural parameter X whichrepresents primary-particle diameter was 0.080 and the value ofstructural parameter Y which represents an isotropic microstrain was0.223.

Using a bead mill, 1 kg of the cerium oxide powder was wet-processpulverized. A fluid containing the pulverized particles obtained wasdried, and the dried particles were ball-mill pulverized. The resultantpulverized particles were observed on a scanning electron microscope tofind that they had been pulverized to particles having the same size asprimary-particle diameter and no large pulverization residue particleswere seen. Precision measurement by X-ray diffraction was made on thepulverized particles, and the results obtained were analyzed by theRietvelt method (RIETAN-94) to find that the value of structuralparameter X which represents primary-particle diameter was 0.085 and thevalue of structural parameter Y which represents an isotropicmicrostrain was 0.300. As a result, there was almost no variation inprimary-particle diameter caused by pulverization and also strains wereintroduced into particles as a result of pulverization. Measurement ofspecific surface area by the BET method also revealed that it was 10m²/g.

(Production of cerium oxide slurry)

1 kg of the cerium oxide particles produced in the above, 23 g of anaqueous ammonium polyacrylate solution (40% by weight) and 8,977 g ofdeionized water were mixed, and the mixture formed was subjected toultrasonic dispersion for 10 minutes with stirring. The slurry thusobtained was filtered with a 1 μm filter, followed by further additionof deionized water to obtain a 3% by weight abrasive. The slurry had apH of 8.3. Particle size distribution of slurry particles was examinedby laser diffraction (measuring apparatus: MASTER SIZER MICROPLUS;refractive index: 1.9285) to find that the median diameter was 290 nm.With regard to maximum particle diameter, particles of 780 nm or largerwere in a content of 0%.

To examine dispersibility of the slurry and charges of the slurryparticles, the zeta potential of the slurry was measured. The ceriumoxide slurry was put in a measuring cell provided with platinumelectrodes on both sides, and a voltage of 10 V was applied to bothelectrodes. Slurry particles having come to have charges uponapplication of the voltage move toward the electrode side having apolarity reverse to that of the charges. The zeta potential of particlescan be determined by determining their mobility. As a result of themeasurement of zeta potential, it was confirmed that the particles werecharged negatively, and showed a large absolute value of −50 mV, havinga good dispersibility.

(Polishing of insulating film layer)

A silicon wafer on which an SiO₂ insulating film produced by TEOSplasma-assisted CVD was formed was set on a holder provided with asuction pad stuck thereon for attaching the substrate to be held, andthe holder was placed, with its insulating film side down, on a platenon which a polishing pad made of porous urethane resin was stuck. Aweight was further placed thereon so as to provide a processing load of300 g/cm². The platen was rotated at 30 rpm for 2 minutes to polish theinsulating film while dropwise adding the above cerium oxide slurry(solid content: 3% by weight) onto the platen at a rate of 35 ml/minute.

After the polishing was completed, the wafer was detached from theholder and then well rinsed in running water, followed by furthercleaning for 20 minutes using an ultrasonic cleaner. After the cleaningwas completed, the wafer was set on a spin dryer to remove drops ofwater, followed by drying for 10 minutes using a 120° C. dryer. Changesin layer thickness before and after the polishing were measured with alight-interference type layer thickness measuring device. As a result,it was found that as a result of this polishing the insulating film wasabraded by 560 nm (polishing rate: 280 nm/minute) and the wafer was in auniform thickness over its whole area. The surface of the insulatingfilm was also observed using an optical microscope, where no evidentscratches were seen.

EXAMPLE 4

(Production of cerium oxide particles)

2 kg of cerium carbonate hydrate was placed in a container made ofplatinum, followed by firing at 700° C. for 2 hours in air to obtainabout 1 kg of a yellowish white powder. Phase identification of thispowder was made by Xray diffraction to confirm that it was cerium oxide.The fired powder had particle diameters of 30 to 100 μm. The particlesurfaces of the fired powder were observed on a scanning electronmicroscope, where grain boundaries of cerium oxide were seen. Diametersof cerium oxide primary particles surrounded by the grain boundarieswere measured to find that the median diameter and maximum diameter intheir particle size distribution were 50 nm and 100 nm, respectively.Precision measurement by X-ray diffraction was made on the fired powder,and the results obtained were analyzed by the Rietvelt method(RIETAN-94) to find that the value of structural parameter X whichrepresents primary-particle diameter was 0.300 and the value ofstructural parameter Y which represents an isotropic microstrain was0.350.

Using a bead mill, 1 kg of the cerium oxide powder was wet-processpulverized. A fluid containing the pulverized particles obtained wasdried, and the dried particles were ball-mill pulverized. The resultantpulverized particles were observed on a scanning electron microscope tofind that they had been pulverized to particles having the same size asprimary-particle diameter and no large pulverization residue particleswere seen. Precision measurement by X-ray diffraction was made on thepulverized particles, and the results obtained were analyzed by theRietvelt method (RIETAN-94) to find that the value of structuralparameter X which represents primary-particle diameter was 0.302 and thevalue of structural parameter Y which represents an isotropicmicrostrain was 0.450. As a result, there was almost no variation inprimary-particle diameter caused by pulverization and also strains wereintroduced into particles as a result of pulverization. Measurement ofspecific surface area by the BET method also revealed that it was 40m²/g.

(Production of cerium oxide slurry)

1 kg of the cerium oxide particles produced in the above, 23 g of anaqueous ammonium polyacrylate solution (40% by weight) and 8,977 g ofdeionized water were mixed, and the mixture formed was subjected toultrasonic dispersion for 10 minutes with stirring. The slurry thusobtained was filtered with a 1 μm filter, followed by further additionof deionized water to obtain a 3% by weight abrasive. The slurry had apH of 8.5. Particle size distribution of slurry particles was examinedby laser diffraction (measuring apparatus: MASTER SIZER MICROPLUS;refractive index: 1.9285) to find that the median diameter was 290 nm.With regard to maximum particle diameter, particles of 780 nm or largerwere in a content of 0%.

To examine dispersibility of the slurry and charges of the slurryparticles, the zeta potential of the slurry was measured. The ceriumoxide slurry was put in a measuring cell provided with platinumelectrodes on both sides, and a voltage of 10 V was applied to bothelectrodes. Slurry particles having come to have charges uponapplication of the voltage move toward the electrode side having apolarity reverse to that of the charges. The zeta potential of particlescan be determined by determining their mobility. As a result of themeasurement of zeta potential, it was confirmed that the particles werecharged negatively, and showed a large absolute value of -65 mV, havinga good dispersibility.

(Polishing of insulating film layer)

A silicon wafer on which an SiO₂ insulating film produced by TEOSplasma-assisted CVD was formed was set on a holder provided with asuction pad stuck thereon for attaching the substrate to be held, andthe holder was placed, with its insulating film side down, on a platenon which a polishing pad made of porous urethane resin was stuck. Aweight was further placed thereon so as to provide a processing load of300 g/cm². The platen was rotated at 30 rpm for 2 minutes to polish theinsulating film while dropwise adding the above cerium oxide slurry(solid content: 3% by weight) onto the platen at a rate of 35 ml/minute.

After the polishing was completed, the wafer was detached from theholder and then well rinsed in running water, followed by furthercleaning for 20 minutes using an ultrasonic cleaner. After the cleaningwas completed, the wafer was set on a spin dryer to remove drops ofwater, followed by drying for 10 minutes using a 120° C. dryer. Changesin layer thickness before and after the polishing were measured with alight-interference type layer thickness measuring device. As the result,it was found that as a result of this polishing the insulating film wasabraded by 400 nm (polishing rate: 200 nm/minute) and the wafer was in auniform thickness over its whole area. The surface of the insulatingfilm was also observed using an optical microscope, where no evidentscratches were seen.

COMPARATIVE EXAMPLE

A silicon wafer on which an SiO₂ insulating film produced by TEOSplasma-assisted CVD was formed in the same manner as in Examples, waspolished using a silica slurry. Silica slurry is one having a pH of 10.3and containing 12.5% by weight of SiO₂ particles. The polishing wascarried out under the same conditions as in Examples. As a result,scratches caused by polishing were not seen, and the insulating filmlayer was polished uniformly, but was abraded only by 150 nm as a resultof polishing for 2 minutes (polishing rate: 75 nm/minute).

Possibility of Industrial Application

As described above, the abrasive according to the present invention canpolish the surfaces of polish objects such as SiO₂ insulating films at ahigh rate without causing scratches, and is especially suited for use inthe polishing of given substrates such as semiconductor chips.

What is claimed is:
 1. A cerium oxide abrasive comprising a slurrycontaining cerium oxide particles comprising plural primary particleswherein: said primary particles consist essentially of cerium oxide;said primary particles have a median diameter of from 30 nm to 250 nm;said cerium oxide particles have a median diameter of from 150 nm to 600nm; and said cerium oxide particles are obtained by pulverizing ceriumoxide obtained by firing cerium carbonate, wherein said primaryparticles have a structural parameter Y, representing an isotropicmicrostrain in analysis by the powder X-ray Rietvelt method (RIETAN-94),of from 0.01 to 0.70.